This invention relates to hot-dip zinc alloy coated steel products having improved resistance to corrosion and good adhesive property of the alloy coatings, to methods of their manufacture and to flux compositions used in the manufacture.
Zinc alloy coatings have long been applied to steel products and compared to electroplating the hot-dip zinc alloy coating can be applied to structurally complicated steel products such as pipes and tubes, section steel, and shaped or contoured parts, such as bolts and nuts, and of course can also be applied to steel sheets and steel wires, etc., at a low cost. For this reason, the use of hot-dipping has become very popular. On comparison with electrogalvanized steel products produced by electroplating, however, hot-dip zinc alloy coated steel products produced by the hot dipping process are found to be inferior in the adhesion of the coating to the steel surface, and for this reason, in workability and further in corrosion resistance.
With the object of improving defects of hot-dip zinc alloy coated steel products, and of improving the corrosion resistance of the coatings, in particular, many propsals have been made on hot-dip zinc alloy coated steel products.
In the Japanese Patent Publication (before examination) No. Tokkaisho 50-104731 (1975), a Zn-Al-Si alloy for hot-dip coating having a composition of 5-20 wt% aluminium (Al), less than 5 wt% silicon (Si) and remainder zinc (Zn) is proposed and in the Japanese Patent Publication (before examination) No. Tokkaisho 54-23032 (1979), a Zn-Al-Si-Sn alloy for hot-dip coating having a composition of 2-20 wt% Al, 0.001-0.5 wt% Si, 0.01-0.1 wt% Sn and remainder Zn is proposed. In both of these coatings, however, highly Al rich bata (.beta.) phase precipitates as primary crystals in the alloy coating and in regions where the Al content exceeds 5 wt% the corrosion resistance deteriorates. In particular, in the case of the latter proposal, in regions where the Al level falls below 5 wt%, the coating shows remarkably favorable corrosion weight loss in a salt water spray test, but by the influence of added Sn, intercrystalline corrosion appears and further adhesion between the coating and the steel product is inferior and the coating peels off by 2T bending test and tape test and consequently, the workability is bad.
The Japanese Patent Publication (before examination) No. Tokkaisho 58-177446 (1983) discloses alloy coated steel sheets having good corrosion resistance and paintability and the coating consists of 3-40 wt% Al, 0.05-2.0 wt% Mg, 0.015-4.0 wt% Si (0.005-0.1 times of Al content), less than 0.02 wt% lead (Pb) and the remainder Zn. Even in this method, in the range where Al content exceeds 5 wt%, corrosion resistance deteriorates because Al--rich and grown beta-phase precipitates as primary crystals. On the other hand when Al content falls below 5 wt%, corrosion weight loss becomes very small in a salt water spray test, however, intercrystalline corrosion occurs since Mg in the alloy coating exceed the solubility limit in solid solution and this Mg serves to cause intercrystalline corrosion, rather than prevent it to meet the purpose for which it is normally added.
On the other hand, there are dry methods and wet method for production of hot-dip alloy coated steel by using a molten bath of zinc alloy. In dry method, non-annealed steel products have their surfaces chemically reduced by hydrogen and ammonia, etc., at a high-temperature in a furnace, and are then dipped in a coating bath at controlled temperature. In the wet method, annealed steel products are degreased by alkali, acid-pickled, rinsed with water sufficiently, and fluxed and then they are dipped in a coating bath. For fluxing, the steel products are ordinally dipped in water solution of flux composition and dried to form the film of flux composition on the steel surface and then the steel product with thin film is dipped in coating bath. In the coating bath, flux composition melts and removes from the surface of steel products to expose the surface to and contact with melted zinc alloy. The dry method is an excellent method if the coating process is connected with steelmaking and rolling sectors, but its application is difficult when coating is applied for steel pipes and tubes, section steel and shaped or contoured steel parts, etc. The wet method can be used either with steel sheets, steel wires, steel pipe and tubes, section steel and shaped or contoured steel part. In the wet method, however, it is difficult to maintain the steel surface in a stable condition as in the dry method, and as a result, alloy coatings have defects such as un-coating. In order to prevent the defect of un-coating, fluxing is performed before coating, but the fluxes generally used, such as ammonium chloride (NH.sub.4 Cl) and ammonium zinc chloride (ZnCl.sub.2.3NH.sub.4 Cl) can be used for hot-dip zinc alloys of any compositions. However, when ZnCl.sub.2.3NH.sub.4 Cl is used for Zn-Al alloy coating, un-coating can not be avoidable.